Catalytic oxidative dehydrogenation of alkanes into corresponding alkenes is an alternative to steam cracking. The latter is the method of choice for the majority of today's commercial scale producers. Despite its widespread use, steam cracking has its downsides.
First, steam cracking is energy intensive, requiring temperatures in the range of 700° C. to 1000° C. to satisfy the highly endothermic nature of the cracking reactions.
Second, the process is expensive owing to the high fuel demand, the requirement for reactor materials that can withstand the high temperatures, and the necessity for separation of unwanted by-products using downstream separation units.
Third, the production of coke by-product requires periodic shutdown for cleaning and maintenance.
Finally, for ethylene producers, the selectivity for ethylene is only around 80 to 85% for a conversion rate that doesn't generally exceed 60%. In contrast, ODH operates at lower temperature, does not produce coke, and using newer developed catalysts provides selectivity for ethylene of around 98% at close to 60% conversion. The advantages of ODH are, however, overshadowed by the requirement for the potentially catastrophic mixing of oxygen with a hydrocarbon.
The concept of ODH has been known since at least the late 1960's. Since that time, considerable effort has been expended on improving the process, including improving catalyst efficiency and selectivity. This has resulted in numerous patents for various catalyst types including carbon molecular sieves, metal phosphates, and most notably mixed metal oxides. Early catalyst U.S. patents assigned to Petro-Tex Chemicals, including U.S. Pat. Nos. 3,420,911 and 3,420,912 in the names of Woskow et al., taught the use of ferrites in the oxidative dehydrogenation of organic compounds. The ferrites are introduced into a dehydrogenation zone containing the organic compound and an oxidant for a short period, then to a regeneration zone for reoxidation, and then fed back to the dehydrogenation zone for another cycle.
U.S. Pat. No. 4,450,313, issued May 22, 1984 to Eastman et al., assigned to Phillips Petroleum Company, discloses a catalyst of the composition Li2O—TiO2, which is characterized by a low ethane conversion not exceeding 10%, in spite of a rather high selectivity to ethylene (92%), using a process where temperature is at or higher than 650° C.
The preparation of a supported catalyst useful for low-temperature oxidative dehydrogenation of ethane to ethylene is disclosed in U.S. Pat. No. 4,596,787, issued Jun. 24, 1986 to Manyik et al., assigned to Union Carbide Corporation. A supported catalyst for the low-temperature gas-phase oxidative dehydrogenation of ethane to ethylene is prepared by (a) preparing a precursor solution having soluble and insoluble portions of metal compounds, (b) separating the soluble portion, (c) impregnating a catalyst support with the soluble portion and (d) activating the impregnated support to obtain the catalyst. The calcined catalyst has the compositionMoaVbNbcSbdXe wherein X is nothing or Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U, Mn and/or W; a is 0.5-0.9; b is 0.1-0.4; c is 0.001-0.2; d is 0.001-0.1; and e is 0.001-0.1 when X is an element.
Other examples of low temperature oxidative dehydrogenation of ethane to ethylene using a calcined oxide catalyst containing molybdenum, vanadium, niobium and antimony are described in U.S. Pat. No. 4,524,236 A, issued Jun. 18, 1985 and U.S. Pat. No. 4,250,346 A, issued Feb. 10, 1981, both assigned to Union Carbide Corporation. The catalyst is prepared from a solution of soluble compounds and/or complexes of each of the metals. The dried catalyst is calcined by heating at 220° C. to 550° C. in air or oxygen. The catalyst precursor solutions may be supported on an inorganic oxide (e.g., silica, aluminum oxide, silicon carbide, zirconia, titania or mixtures of these). The selectivity to ethylene may be greater than 65% for a 50% conversion of ethane.
U.S. Pat. No. 4,524,236 issued Jun. 18, 1985 to McCain assigned to Union Carbide Corporation and U.S. Pat. No. 4,899,003, issued Feb. 6, 1990 to Manyik et al. assigned to Union Carbide Chemicals and Plastics Company Inc. disclose mixed metal oxide catalysts of V—Mo—Nb—Sb. At 375° C. to 400° C., the ethane conversion reached 70% with the selectivity close to 71 to 73%. However, this ethane conversion result was only achieved at very low gas hourly space velocities (i.e., 720 h−1).
U.S. Pat. No. 6,624,116, issued Sep. 23, 2003 to Bharadwaj, et al. and U.S. Pat. No. 6,566,573 issued May 20, 2003 to Bharadwaj, et al., both assigned to Dow Global Technologies Inc., disclose Pt—Sn—Sb—Cu—Ag monolith systems that have been tested in an auto-thermal regime at T>750° C. where the starting gas mixture contains hydrogen (H2:O2=2:1, gas hourly space velocity (GHSV) of 180 000 h−1). The catalyst composition is different from that of the present disclosure and the present disclosure does not contemplate the use of molecular hydrogen in the feed.
U.S. Pat. No. 6,521,808 issued Feb. 18, 2003 to Ozkan, et al, assigned to Ohio State University teaches sol gel supported catalysts for the oxidative dehydrogenation of ethane to ethylene. The catalyst appears to be a mixed metal system, such as, Ni—Co—Mo, V—Nb—Mo possibly doped with small amounts of Li, Na, K, Rb, and Cs on a mixed silica oxide/titanium oxide support. The catalyst does not provide the oxygen for the oxidative dehydrogenation, rather, gaseous oxygen is included in the feed. The patent does not teach the integration of oxygen separation from air using by-products from the ODH reaction catalyzed by the claimed catalyst.
U.S. Pat. No. 6,891,075 issued May 10, 2005 to Liu, assigned to Symyx Technologies, Inc. teaches a catalyst for the oxidative dehydrogenation of a paraffin (alkane) such as ethane. The gaseous feedstock comprises at least the alkane and oxygen, but may also include diluents (such as, argon, nitrogen, etc.) or other components (such as, water or carbon dioxide). The dehydrogenation catalyst comprises at least about 2 weight % of NiO and a broad range of other elements, preferably, Nb, Ta, and Co. The claims required a selectivity for ethylene of at least 70%, with conversions over 10%.
U.S. Pat. No. 7,319,179 issued Jan. 15, 2008 to Lopez-Nieto et al. assigned to Consejo Superior de Investigaciones Cientificas and Universidad Politecnica de Valencia discloses Mo—V—Te—Nb—O oxide catalysts that provided an ethane conversion of 50 to 70% and selectivity to ethylene up to 95% (at 38% conversion) at 360 to 400° C. The catalysts have the empirical formula MoTehViNbjAkOx, where A is a fifth modifying element. The catalyst is a calcined mixed oxide (at least of Mo, Te, V and Nb), optionally supported on: (i) silica, alumina and/or titania, preferably silica at 20 to 70 wt. % of the total supported catalyst or (ii) silicon carbide. The supported catalyst is prepared by conventional methods of precipitation from solutions, drying the precipitate and then calcining.
There is also considerable prior art directed at the process of oxidative dehydrogenation itself, including reactor configurations and implementations for increased safety.
U.S. Pat. No. 3,904,703 issued Sep. 9, 1975 to Lo et al., assigned to El Paso Products Company, teaches a zoned or layered oxidative reactor in which following a zone for oxidative dehydrogenation there is an “oxidation zone” to oxidize the hydrogen to water. Following the oxidation zone, there is an adsorption bed to remove water from the reactants before they enter a subsequent dehydrogenation zone. This is to reduce the impact of water on downstream dehydrogenation catalysts.
EP Patent 0 261 264 B1, inventors Manyik, et. al., assigned to Union Carbide Corporation, granted Aug. 21, 1991, discloses the use of multiple reactors in series, each reactor followed by a cooling step and the addition of supplemental oxygen before each subsequent reactor. The effect of interstage cooling and addition of supplemental oxygen prior to each reactor except the final reactor is to reduce total water and acetic acid content prior to each stage.
U.S. Pat. No. 5,202,517 issued Apr. 13, 1993 to Minet et al., assigned to Medalert Incorporated, teaches a ceramic tube for use in the conventional dehydrogenation of ethane to ethylene. The “tube” is a ceramic membrane in which the ethane flows inside the tube and hydrogen diffuses out of the tube to improve the reaction kinetics. The reactive ceramic is 5 micrometers thick on a 1.5 to 2 mm thick support.
U.S. Published Application No. 20110245571 in the name of NOVA Chemicals (International) S.A. teaches oxidative dehydrogenation of ethane in a fluidized bed in contact with a bed of regenerative oxides to provide oxygen to the reactor. In this process, free oxygen is not directly mixed with the feedstock reducing the likelihood of decompositions.
U.S. Published Application No. 20180009662, filed in the name of NOVA Chemicals (International) S.A., titled “Inherently Safe Oxygen/Hydrocarbon Gas Mixer”, inventor Vasily Simanzhenkov et al., discloses the use of a flooded mixer, where hydrocarbon and oxygen containing gases are mixed without fear of igniting an explosive event. This benefit is directly applicable to ODH as it provides an option for improving the safety of the process. Gases, in a ratio that falls outside of the flammability envelope, can be mixed together before introduction into an ODH reactor. This patent does not teach the incorporation of oxygen separation into the ODH process.
None of the above art teaches or suggests a chemical complex in which the by-products of the ODH process are used to drive an oxygen separation process, resulting in a relatively pure source of oxygen which can be recycled back to contribute to the ODH reaction.
In order for ODH to become a mainstream commercial option the economic benefit must outweigh the risk associated with thermal runway of the reaction. Most prior art patents are directed at improving the safety and efficiency of the reaction by developing better catalysts and systems for reducing risk. The present disclosure seeks to further improve economic efficiency by reducing the amount of costly oxygen required during steady state operations. Furthermore, the use of ODH by-products to drive oxygen separation limits the amount of carbon dioxide that would be released into the atmosphere.